1. Field of the Invention
This invention relates generally to scale abatement and, more particularly, this invention relates to a method of reducing or substantially eliminating the rate and extent of formation of alkaline scale on cooling tower surfaces and other heat transfer surfaces which are contacted by circulating water cooled in cooling towers.
2. Brief Description of the Prior Art
Much research has been directed to the elimination or reduction of alkaline scale formation during evaporative processes operating on water which contains dissolved salts of magnesium and/or calcium. For example, in all types of sea water distillation equipment, heating of sea water to a temperature up to about 180.degree. F. produces scale which is predominately calcium carbonate (CaCO.sub.3). At temperatures above about 200.degree. F., magnesium hydroxide [Mg(OH).sub.2 ] scale predominates. At temperatures between about 180.degree. F. and 200.degree. F. either type of scale, or mixtures thereof, may be encountered. Calcium carbonate and magnesium hydroxide scales are collectively referred to as alkaline scales.
Alkaline scale formation on surfaces of cooling towers and on heat exchange equipment contacted by cooling tower water has, similarly, long been a major problem. Since scale is a heat insulator, formation of alkaline scale tends to inhibit heat transfer from the apparatus being cooled by the circulating water stream. Accumulation of scale on wetted cooling tower surfaces (which are provided to achieve evaporative cooling through intimate contact with a moving air stream) interfere with efficient operation of cooling towers. Scale accumulation may result in shut downs, the cost of which is substantial, as is the direct cost of cleaning scaled surfaces.
For many years, a common means of controlling alkaline scaling in cooling towers has been to destroy bicarbonates and carbonates present in the water by adding an acid, such as a mineral acid, preferably sulfuric acid. Sulfuric acid has commonly been used because of its effectiveness and because it is usually the most economical source of hydrogen ions to produce the required chemical reactions.
Cooling towers using acid as a means of controlling alkaline scale formation normally operate at a pH of about 6-7. Such an acidic pH range is corrosive to iron and copper alloy surfaces contacted by the cooling water. In order to inhibit corrosion of these metals it has become common practice to add chromates and polyphosphates, either singly or in combination, and often with a source of metal ions such as zinc.
Recently, however, it has been recognized that these corrosion inhibiting chemicals, which are present in the cooling tower blowdown stream, present an environmental pollution problem. Since non-polluting corrosion inhibitors have not yet been found which are effective under the acidic operating conditions utilized in some cooling towers, operation of cooling towers in alkaline pH ranges has been attempted. At alkaline pH levels, corrosion rates can be controlled using available chemicals at dosage levels which have resulted in relatively low levels of pollution. However, operation under alkaline conditions requires scale inhibitors which are effective under alkaline pH conditions, because such conditions favor the precipitation of calcium carbonate and, if magnesium is present in the water, magnesium hydroxide.
Pollution and water supply conservation concerns have been an incentive to operate cooling towers at relatively high dissolved salt concentrations in order to minimize the volume of the cooling tower blowdown stream which must be disposed of, and to minimize the volume of makeup water needed which in many cases is pretreated prior to its introduction into the cooling tower system. Operation at high salt concentrations, however, results in higher pH levels and hardness concentrations so that, in many cases, sulfuric acid is added to reduce the pH and the total alkalinity of the cooling tower water in order to reduce the severity of the chemical conditions existing, thus enhancing of the effectiveness of alkaline scale inhibitors.
However, the use of sulfuric acid as noted above presents a secondary scale problem which is a result of the limited solubility of calcium sulfate. Since most water supplies contain significant concentrations of calcium and since sulfate ions are introduced to cooling tower water by the addition of sulfuric acid, care must be taken to withdraw enough cooling tower blowdown to avoid supersaturation and deposition of one or more of the various crystalline forms of calcium sulfate. In some cases, threshold type chemical additives are employed to permit operation of cooling towers under conditions in which calcium sulfate scales would otherwise occur.
To date, none of the means of controlling alkaline scale formation summarized above have been totally satisfactory, as such approaches generally result in high corrosion rates or high dosing rates of threshold effect scale control additives, among other problems.